i) Field of the Invention
This invention relates to contaminant detection in paper pulp, and particularly to a device for detecting and measuring the concentration of lightweight, oversize contaminant particles in virgin pulp and recycled pulp. The invention also relates to an apparatus for separating the lightweight contaminants from pulp and isolating the lightweight, oversize contaminants.
ii) Description of Prior Art
In the manufacture of paper products, an appropriate fibre pulp is used which may be purchased in the form of solid pulp sheets, or processed in liquid form. It has long been known that the resulting paper products are greatly affected by the inclusion of foreign matter in the basic pulp from which the paper product is made. Common contaminants such as dirt, pitch, fibre bundles (identified in the art as shives), and the like are the usual contaminants encountered in virgin pulp. Various means have been provided for the detection of these contaminants.
Certain contaminants, such as lightweight plastic, can cause serious problems in the production of various grades of paper at much lower concentrations than common contaminants. See, for example, Enchin H. "Plastic Places Paper Sales at Risk", The Globe and Mail (1994 Apr. 14), p. B5, Westergaard B., "Plastic Contamination Costing Pulp Mills Dearly", Pulp and Paper J. 37(1):74-78, (1984); and Robitaille M. "Plastic Contamination in the Pulp Mill: an Overview", Pulp and Paper Can. 89(1):121-125 (1988). In the production of coated paper, for example, a blade spreads the coating medium over a fast-moving, continuous paper sheet. A plastic speck protruding from the surface of the sheet can catch on the blade and obstruct the flow of the coating medium. A streak of uncoated paper will result and the entire reel of paper will be rejected. The speck can also cause the sheet to tear, and lost production will result. In other paper grades, plastic specks can interfere with the transfer of ink or surface treatments to the paper. In tissue manufacture, plastic specks can stick to the highly-polished dryer cylinder and cause sheet tears.
An accurate and convenient method is required to inspect the pulp for plastic specks before it is made into paper. Contaminated pulp can then be redirected for use in paper grades which are not sensitive to plastic contaminants. Accurate measurements of plastic levels are also useful in identifying operating procedures or equipment which reduce the level of contamination- The measurements can also give an early warning of any process upsets or equipment failures which have caused contaminant levels to increase.
The most commonly used method for measuring plastic levels is the inspection of dried pulp sheets. This is the form the pulp is typically in when it leaves the pulp mill to go to the paper mill. The sheet inspection procedure is manual and uses transmitted and/or reflected light. The pulp sheet may be wetted to make the pulp more transparent. There are, however, serious flaws with this method: 1) it is subjective, 2) clear and white plastic specks are difficult to see against the white pulp background, and 3) the amount of pulp inspected is too small to give a statistically sound measurement of plastic contaminants.
Some other measurement techniques are based on intermittently sampling a small amount of the pulp while it is in aqueous suspension, and manually passing the suspension through a lab-type pulp screen, such as the Somerville Fractionator, Pulmac Shive Analyzer or Valley Flat Screen. This approach is more flexible, inasmuch as the pulp samples may be taken at various points in the process, either at the pulp mill or paper mill, but it has drawbacks that make it less attractive than the sheet-inspection procedure described above: 1) It is less convenient because the sample must be brought to a lab to be analyzed. 2) Considerable amounts of pulp fibre may be retained on the screen with the plastic contaminants. The pulp must therefore be dissolved using a chemical treatment, or the specks must be isolated manually. 3) Most seriously, the amount of pulp sampled is less than that obtained in the sheet-inspection approach, which is already too little to give a statistically sound measurement.
Several methods have been developed which sample the pulp suspensions in sufficient quantities to give a statistically sound measurement of plastic contaminants, but they have found only limited use. One is the Gyroclean (Trade Mark) which is a device that concentrates lightweight contaminants using a centrifuge with continuous flow-through. (See, for example, Marson M. "New Lightweight Concentrator Units Solve Mill's Plastic Problems", Pulp and Paper 93-96 (June 1990); and Julien Saint Amand J. "Process and Device for Separating Particles in a Fluid Especially for the Cleaning of the Suspensions Handled in the Paper Industry", U.S. Pat. No. 4,443,331 (1994)). One limitation of this approach is that this equipment is very costly to purchase and maintain. Another problem is that the incomplete separation of fibre and plastic contaminants will require that the sample be brought to the lab for additional treatments to isolate the plastic specks.
Another method for measuring plastic contamination of pulp is described by Robitaille M.A., Peeling D.A. "Light Weight Contaminant Monitoring System", 1987, CPPA Gadget Competition p.59-61. With this approach, a continuous stream of pulp is sampled from the process line and passed through a centrifugal device to concentrate the lightweight plastic particles. The stream with lightweight particles then passes to a settling tank. Lightweight plastic specks are intended to float to the surface of the tank, where they can be skimmed off to give a measure of plastic contamination levels. There are two main problems with this approach. First, the buoyant forces of certain plastic specks may be too small to allow the floatation cell to work efficiently and these specks will not be collected by the floatation cell. Second, the surface of the floatation tank may be obscured by steam, foam and pulp, and the collection of isolated plastic specks will become very difficult.
The contaminant detection method of Carr W. F. "System for Monitoring Contaminants with a detector in a Paper Pulp System", U.S. Pat. No. 4,758,308 (1988), is similar to that described above inasmuch as a continuously sampled stream of pulp is removed from the process stream, and passed through a centrifugal device to concentrate the contaminants. It differs, however, inasmuch as the contaminants are quantified not by isolating them for manual inspection, but by passing a dilute suspension of the contaminants past an electronic camera. The camera images can be analyzed to produce an electronic signal that indicates the level of contamination in the suspension. The problem with this approach, however, is that light imaging of the contaminants would not be expected to distinguish between plastic contaminants and other, more common, but less harmful contaminants. Plastic contaminants exist at much lower concentrations than pitch, shives, talc balls, and other common contaminants. Conclusive identification of plastic requires off-line inspection. Collection of the plastic specks also offers the advantage of being able to archive specks for further analysis and comparison.
The detection apparatus of Hayakawa, U.S. Pat. No. 3,709,614 is like that of Carr above inasmuch as contaminants are concentrated from a flowing suspension and passed to an inspection cell where an electronic camera assesses the contaminant concentration. The principal difference is in the use of screens instead of centrifugal devices to concentrate the contaminants. The use of the electronic camera, however, leads this apparatus to share the limitations which were identified with the detection method of Carr.
There is thus a very significant demand in the art for a method and device for detecting and measuring contaminant particles, particularly lightweight plastic particles. The system should preferably sample the pulp continuously to provide a large and representative pulp sample. It should efficiently remove and isolate the specks for quantification and analysis, be of low cost, and provide a convenient and quick measurement of contaminant concentration.